Cannot debug external_flash example

I have downloaded external_flash example from https://github.com/circuitdojo/nrf9160-feather-examples-and-drivers/blob/v1.7.x/ and run it without any modification. The example can erase and flash but it would not start in main() or hit any breakpoint set.

What am I missing?

My setup is Sparkfun Thing plus nRF9160 with nRF Connect SDK v1.7.0, SEGGER Embedded Studio V5.6

How are you programming your board? nrfjprog? Are you flashing the merged.hex file?

thanks for the awesome information.